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Women are a vital part of the housing industry — moving markets and encouraging those around them. Rising interest rates often cause homebuyers to think twice about moving forward with a mortgage. The same kind of cold gas is also present in galaxies of lower mass and to even larger impact parameter. Jia Liang and Chen search for galaxy—QSO close pairs suitable for finding gas absorption systems. The above results present just a few cases among the extensive list of evidence for neutral gas reservoirs next to galaxies drawn from absorption on QSO spectra — more will be given in Sect.
Within the cold-flow accretion scenario, gravitationally bound HI structures devoid of stars are to be expected. HI filaments of this kind have been discovered in blind HI surveys. For example, Popping and Braun a analyze data from the Parkes All Sky Survey to find ten faint extragalactic filaments that can correspond to extended haloes, tidal remnants, or potentially diffuse filaments tracing the neutral fraction of the cosmic web.
Popping and Braun b map the galaxy filament connecting the Virgo cluster with the local group, finding 20 new detections of neutral hydrogen with no obvious sign of stellar emission. Other examples exist in the recent literature e. So far it is unclear whether the small number of HI-only sources agrees with cosmological numerical simulations since the predictions are not specific enough. Environment influences gas content and recycling. The most likely interpretation is that the early-type dwarfs who were once stripped in the cluster are recently re-accreting gas. In a VLA HI study of counter-rotating disks, three early-type barred galaxies appear to be accreting cold gas.
Several of the HI features are cold gas blobs, possibly gas-rich dwarfs Chung et al. Kreckel et al. While some of the galaxies are interacting or have companions, others appear isolated and undisturbed, with flocculent spiral or chain morphologies. They note that one void galaxy is a polar-ring galaxy in a thin wall between voids, and may be slowly accreting gas from the cosmic web, as also for another ring galaxy in a void Spavone and Iodice Two dwarf galaxies with extended HI disks have unusual kinematics that may be explained by ongoing cold-flow accretion.
To characterize faint diffuse gas in local galaxies, Heald et al. Their deep HI survey is designed to search for extraplanar gas in edge-on and inclined nearby galaxies and to determine the distribution and gas properties. The pilot survey of four galaxies tentatively suggests that galaxies with higher star-formation energy form gas haloes from outflow, and that extraplanar gas is associated with a high star-formation rate per unit area Zschaechner et al.
The full survey should also be a useful characterization of halo gas in local systems. Morphological distortions. Very often the HI maps present non-axisymmetric distortions showing that the gas is not contained in a disk or a spheroid e. For example, spirals are known to have extended warped HI distributions, a pattern that cannot be sustained for long. Among other explanations, gas infall has been proposed as the origin of warped disks e.
Such extreme distortion is actually common among galaxies with large specific SFR in the local universe—blue compact dwarf BCD galaxies often show such complex HI topology Ekta et al. Many other examples of HI maps showing galaxies with plumes and filaments can be found in the literature e. Morphological distortions suggestive of gas accretion are also common in optical images and are discussed in Sect. A particularly telling deviation from axisymmetry has been found recently by Kacprzak et al.
They report a bimodality in the azimuthal angle distribution of low ionization gas around galaxies as traced by MgII absorption along QSO lines-of-sight. The circum-galactic gas prefers to lie near the projected galaxy major and minor axes. The bimodality is clear in blue star-forming galaxies whereas red passive galaxies exhibit an excess of absorption along their major axis. These results suggest the bimodality to be driven by gas accretion along the galaxy major axis and gas outflows along the minor axis Sect. There is no other clear alternative. For this interpretation to be correct, the inflow has to be highly anisotropic, concentrated in the plane of the galaxy.
This fact favors cold-flow accretion with respect to the isotropic gas inflow to be expected from hot coronal gas cooling down Sect. For edge-on galaxies the absorption is more concentrated along the minor axis. This is consistent with the idea that bipolar outflows induced by star formation produce the metallic gas in the haloes. Having low metallicity can be regarded as the main fingerprint of cosmic web gas Sect. Gaseous systems observed in absorption along QSO lines-of-sight are easier to detect if they are metallic, simply because the absorption features are intrinsically stronger.
To avoid this bias, Lehner et al. Interestingly, they find a bimodal metallicity distribution with metal-poor and metal-rich branches peaking at 0. Both branches have a nearly equal number of absorbers. These two populations fit in well with the scenario depicted in Sect. In a complementary work, Jia Liang and Chen find that the absorbing gas next to the galaxies has metallic lines, but only hydrogen absorptions show up at distances beyond the virial radius.
This pattern is corroborated by other studies e. Extremely metal-poor XMP galaxies provide another piece of evidence for large amounts of cosmic gas around local star-forming galaxies. These galaxies have a number of properties consistent with disks being assembled by accretion of gas Elmegreen et al. The CGM has also been studied in absorption in the stellar spectra produced by the host galaxy.
Then, one can unambiguously measure whether the gas goes in or out see Sect. Lebouteiller et al. The HI gas pool around it has a metal abundance lower by a factor of two as compared to the HII regions, and it may even present pockets of gas with metallicity essentially null. The HII metallicity samples the gas in the disk, which is already extremely low in this particular galaxy.
Since the observed HI gas metallicity is at the level expected for the cosmic web Sect. Gas-consumption timescales. The study is based on thousands of objects so it portrays a general property of galaxies. The fact that galaxies have been forming stars along the Hubble time e. The emission line spectra produced by HII regions surrounding the star-forming regions provide a direct means of measuring the physical conditions of the gas forming stars and, in particular, its metallicity that represents the prime diagnostic for cosmic gas accretion Sect.
There are a number of compelling observations of ongoing gas accretion based on such HII-based metallicity measurements. Metallicity inhomogeneities and inverted gradients. The secular evolution of disk galaxies produces a regular pattern with the metallicity decreasing inside out, i. The timescale for gas mixing in a disk inside an annulus is fairly short, on the order of a rotational period or a few hundred Myr Sect. Deviations from negative metallicity gradients are usually attributed to the recent arrival of cosmic gas that feeds the star formation.
If the gas is accreted through the cold-flow mode Sect. Alternatively, the external gas streams may fuel the disks with metal-poor gas, so that gas mass builds up developing starbursts through internal gravitational instabilities e. In any case, the cold-flow accretion is bound to induce metal-poor starbursts.
Top Images of three tadpole galaxies characterized by having a bright peripheral clump on a faint tail. Bottom Oxygen abundance variation across the galaxies on top. The vertical solid line represents the center of rotation, whereas the vertical dotted lines indicate the location of maxima in SFR. Note the existence of abundance variations, with the minima coinciding with the regions of largest SFRs.
The thick horizontal solid line indicates the solar metallicity. Secular evolution produces negative metallicity gradients, but some spirals show reverse gradients, where the metallicity is lowest in the inner galactic regions. Examples are given by Queyrel et al. This observation may be due to averaging out metallicity drops in inner regions, or other artifacts caused by the limited angular resolution e. Yuan et al. Alternatively, it may also be interpreted as produced by fast inflows within the disk giving gas to the central regions.
Metal-poor gas deposited in the outskirts can be transported outside-in by instabilities or some type of tidal interaction Combes , ; Elmegreen et al. The same mechanism of gas transport is able to explain the presence of metal-poor gas in the narrow-line region of a nearby QSO found by Husemann et al. It may also account for the finding by Moran et al.
Surprisingly, the magnitude of the outer drop is correlated with the fractional HI content of the galaxy. The recent stellar mass growth at the edge of the galaxies is apparently due to the accretion or radial transport of gas from beyond the stellar disk. High metallicity of quiescent BCDs. BCDs are high surface brightness targets relatively easy to detect.
The luminosity of these galaxies is dominated by one or several young starbursts. However, most if not all BCDs contain host galaxies with old stars too e. The dominant starburst is so intense that it cannot be sustained for long; therefore, the BCDs have to be in a transient phase. Consequently, there must be many local galaxies in the pre or post BCD phase, i.
The BCD host galaxies should show up best outside of their starburst regions. They turned out to be rather common: one out of three local dwarf galaxies is of this kind, and there are some thirty of them per BCD galaxy. Their main properties, including their luminosity functions, are consistent with the BCDs being QBCDs observed during a starburst phase in a duty cycle where the quiescent phase lasts 30 times longer than the active phase.
This interpretation presents a difficulty, though: the gas-phase metallicity of the QBCDs is systematically higher than the metallicity of the BCDs. This cannot happen in a closed-box evolution, where the precursor galaxy always has lower metallicity than the follower, so that QBCDs could not be precursors of BCDs. The problem naturally disappears if almost every BCD phase is preceded by the advent of fresh metal-poor gas that triggers the star-formation episode. The stars of BCDs and QBCDs are statistically the same because only a small fraction of galaxy stellar mass is produced in each starburst.
These findings are consistent with the recent results by Zhao et al. Metallicity threshold. They are usually called extremely metal poor XMP. The recent compilation by Morales-Luis et al. Despite repeated efforts to find galaxies more metal poor e. Several explanations have been put forward to account for this minimum metallicity: the self-enrichment of the HII region used for measuring Kunth and Sargent , the metal abundance of the proto-galactic cloud Kunth and Lebouteiller , the metallicity threshold set by the ejecta from population III stars Audouze and Silk ; Thuan and Izotov , technical difficulties for metallicity determinations below a threshold Papaderos et al.
None of them seem to be fully compelling. However, the accretion scenario provides a natural explanation for this long-lasting problem. Numerical simulations predict the cosmic web gas to accumulate metals from the outflows of dwarf galaxies Sect. These contributions add up along the Hubble time so that at redshift zero the cosmic web metallicity has to be at the few percent level Sect. This is the metallicity to be expected if the SF in XMPs is driven by gas directly accreted from the cosmic web.
Nitrogen and Oxygen in green-pea GP galaxies. GPs are star-forming galaxies which receive this name because of their compactness and green color in SDSS composite images Cardamone et al. They have some of the highest specific SFRs in the local Universe, able to double their stellar masses in a fraction of Gyr. Then the mixing with metal-poor gas reduces the metallicity i.
We note that GPs are not special but just extreme cases in the continuous sequence of local star-forming galaxies e. The most lopsided galaxies have a metallicity deficit of 0. The deficit at low mass is greater than at high mass. SDSS color images of polar-ring galaxies from Combes et al. The scale bar is arcsec long, and the rings are indicated with small arrows to guide identification. The central object is usually an early-type gas poor galaxy, but the ring itself is gas rich.
Some galaxies have giant HI polar disks with very weak stellar counterpart e. Often galaxy disks show a number of kinematically distinct components like counter-rotating bulges Prada et al. These features are expected from numerical simulations of minor mergers e. Algorry et al. They produce counter rotating stars that are not dragged along with the gas but mostly produced in situ. Tidal streams or little tails are also kinematically distinct components, and they turn out to be quite common in the local universe.
They are thought to be leftovers of tidally disrupted gas-rich satellites on their way to reach the center of the global gravitational potential. A study by Matthews et al. The highly organized distribution of satellites surrounding the MW and M31 has been long noticed e. The best explanation seems to be that the dwarf satellite galaxies fell into the gravitational potential along only one or two filaments Li and Helmi ; Angus et al. Using a high-resolution dark-matter simulation of the local group, Libeskind et al. These conclusions are confirmed in other simulations of M31 dwarfs Goerdt and Burkert ; Sadoun et al.
Star-formation history of dwarf galaxies. The variation of the SFR with time is often very bursty in late type galaxies, even if they are isolated. Episodes of large star formation are intertwined with long quiescent phases. This result is inferred using fitting techniques on integrated galaxy spectra e. Convulsive SF histories fit in well the cold-flow accretion scenario.
Models predict the process to be intermittent, so if gas accretion drives star formation, then the SFR is expected to be bursty Sect. A number of factors conspire to make the effect more clear in dwarfs. They are actively forming stars at present, so that the bursts are young and luminous, and they provide SF histories with good time resolution. Outflows are far more important in dwarfs, and they quench star formation forcing long inter-burst periods.
Forming massive galaxies by cold-flows requires the contribution of many discrete accretion events; therefore, statistical fluctuations in their arrival time tend to cancel, giving rise to a smooth SF history. The lower the galaxy mass the less effective the statistical averaging, and dwarfs tend to have a more spasmodic SFR. Even though individual HII regions last for a few Myr e. G dwarf problem. There is a long-known deficit of sub-solar metallicity G dwarf stars in the solar neighborhood van den Bergh ; Schmidt ; Lynden-Bell Among them, a continuous metal-poor gas inflow feeding star formation seems to be the preferred one Edmunds , The argument dates back to Larson , who pointed out that star-formation maintained by metal-poor gas accretion self-regulates to produce a constant gas-phase metallicity close to the stellar yield, which implies close to the solar metallicity Sect.
The enhanced deuterium fraction in the Galaxy is also consistent with this scenario e. A number of observational properties characterizing large samples of local star-forming galaxies can be explained if the star-formation is driven by metal-poor gas accretion. The gas infall explanation provides a simple physical unifying mechanism, even though often it is not the only explanation of each individual observation. The very existence of general laws or trends implies that the underlying mechanism has to be something fundamental, since it affects not just a few objects but the bulk of the star-forming galaxies.
Some of these general properties are discussed elsewhere in this work and will not be repeated here; in particular, the short gas-consumption timescale compared with the age of the stars Sect. SFRs are color coded as indicated in the inset. Given a galaxy mass, galaxies of lower metallicity present higher SFR. Right oxygen abundance vs SFR for galaxies of the same mass.go to site
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The stellar mass is color coded according to the inset. Given a SFR, galaxies of lower metallicity have lower mass. The relationship tends to saturate at the high-mass end, where all galaxies have the same metallicity independently of the mass and SFR. Several recent numerical and analytic works explain the FMR within this framework of gas infall forcing SF. It is important to acknowledge that such an explanation implies a time variable infall rate, which moves the galaxy system out of the stationary-state.
Brisbin and Harwit describe the FMR with an intrinsically bursty model which is always out of equilibrium. Galaxies receive metal-poor gas parcels of different masses that are instantaneously mixed up with the existing ISM. This arrival dilutes the metallicity of the gas and triggers star formation, with the two parameters set by the received gas mass and thus correlated. Dayal et al. Both inflows and outflows are required to reproduce the observed relationship, but the outflows dominate by far. Lilly et al. Although more sophisticated, it is very much in the spirit of the simple description given in Sect.
The model galaxy is near the stationary state, but the gas supply available to form stars is allowed to change with time. This drives the system out of the stationary state and provides a dependence of the metallicity on the SFR and mass gas. The authors successfully fit the FMR by Mannucci et al. As in the case of Dayal et al. The full present-day population is shown in grey for reference. Galaxies move up and to the right as they gather more mass and reduce their outflows.
In addition, sudden metallicity drops are produced by the arrival of cosmic gas ready to form stars. Forbes et al. The scatter arises from the intrinsic scatter in the accretion rate, but may be substantially reduced depending on the timescale on which the accretion varies compared to the timescale on which the galaxy loses gas mass.
They show that observational constraints on the scatter can be translated into constraints on the galaxy-to-galaxy variation in the mass loading factor, and into the timescales and magnitude of stochastic accretion onto star-forming galaxies. The parameterization of the FMR by Mannucci et al. Variations are expected if the relationship is caused by cosmic gas accretion since accretion rates and IGM metallicities change over time. Thus, the variation with redshift of the FMR provides a powerful diagnostic tool to evaluate the evolution of the accreted gas along the history of the universe Forbes et al.
This lack of SFR dependence is consistent with the relation between metallicity and SFR being maintained by episodic metal-poor inflows. The advent of fresh gas triggers star formation and drops the metallicity, but it does not change the pre-existing relative abundance between metals.
Bothwell et al. The authors find the HI-based relationship cleaner, and without the saturation at high masses, where irrespectively of their SFR, all massive galaxies have the same metallicity Fig. A similar anti-correlation between HI mass excess and metallicity has been found by Robertson et al. Hughes et al. Interestingly, the relationship is nearly invariant to the environment when going from clusters to the field. This finding indicates that internal evolutionary processes, rather than environmental effects, shape the observed relationships.
According to the theoretical models put forward to explain the FMR in Sect. Thus, the FMR and the relationship discussed in this section are just two renderings of the same underlying physical principle, namely, that galaxies are driven out the equilibrium mass—metallicity relationship by sudden bursts in the inflow rate. The metallicity changes by 0. The authors discard biases due to the finite size of the central region used to estimate metallicities, and to the Hubble type dependence of the radius. This non-trivial observational result is, however, a natural outcome of the gas accretion driven SF process.
However, the gravitational binding energy also depends on the distance of the matter to the center of the gravitational well. At fixed mass, winds escape easier from larger galaxies. Thus, they are expected to have larger mass loading factors. A factor 2 increase in size is equivalent to a factor 2 decrease in mass, and this scaling is consistent with the observed 0. Most of the observational evidence for gas accretion put forward so far has been concentrated on the low-redshift universe. Theory predicts that cold gas accretion is particularly important at high redshift, when DM haloes are low mass and the accreted cosmic gas remains cold and ready to form stars Sect.
Thus, the high-redshift universe corresponds to an epoch of galaxy assembly and high SFR. Here, we review observational evidence emphasizing the high-redshift aspects not addressed elsewhere in the paper. We include a section on the role of mergers, just to complement the statement made in Sect. SFR density dark orange-shaded region and luminosity density blue-shaded region in the universe over the last The left and right axes refer to the SFR and the luminosity, respectively.
Various symbols correspond to various sources in the literature as detailed in the paper by Bouwens et al. For clarity, the curves are offset vertically, successively by 0. Note the clear trend for galaxies with larger present-day stellar mass to have formed their stars earlier.
The figure was taken from Heavens et al. Both the global history Fig. The agreement with the global SF history Fig. At low redshift there is little DM available. The cosmic star-formation history is well reproduced by cosmological numerical simulations including baryons. For example, Schaye et al. After a large number of simulations that comprise the OverWhelmingly Large Simulations Project, theoretical diagrams match the observations. The resulting SFR is limited at high redshift by the build-up of dark-matter haloes; it reaches a broad maximum at intermediate redshift, and then decreases at lower redshift.
The decrease at late times is due to the quenching of SF by lower cooling rates in hotter and lower density gas, in addition to gas exhaustion and self-regulated feedback from stars and black holes. Similar results are provided by, e. Schaye et al. However, it is not yet clear whether feedback from massive stars can solve the problem. The role of stellar feedback through winds and SNe is an active topic of research at this moment.
Galaxies of different masses follow very different SF histories Fig. Low-mass galaxies have had a more uniform SFR, a fact also well captured by simulations e. Gas accretion and feedback, rather than mergers, are the primary processes responsible for the evolution of the cosmic SFR density. This is clear from numerical simulations which point out that accretion of cold gas is the dominant growth mechanism by about an order of magnitude over mergers e. Wang et al. Observations seem to favor this view too.
Glazebrook et al. Bauer et al. If mergers were responsible for the enhanced star formation, observations should show double nuclei and tidal features, which are indeed observed in some galaxies but not in these clumpy types Elmegreen et al. The negative gradient is consistent with regular inside-out star formation with no excess of metal-poor gas in the inner regions.
Shim et al. Their continuous star-formation rate, relative isolation, and number density are consistent with star formation driven by gas infall. Conselice et al. This holds for the most massive galaxies, but the need for gas accretion is even more extreme for less massive galaxies. Based on HST imaging, Kaviraj et al. In marked contrast to the assembly history of massive ellipticals, mass growth is not limited to large radii. The mass growth takes place in a fairly uniform way, with the galaxies increasing their mass at all radii. None of these features reflect the dramatic effects expected from major mergers.
Another indication disfavoring mergers comes from the increase of tadpole or cometary galaxies in high-redshift fields. These galaxies, with a bright head on a faint tail, exist in the local universe as well as at high redshift van den Bergh et al. Local tadpoles are unusual but, surprisingly, they turn out to be very common among extremely low metallicity dwarfs. These and other properties indicate that they are disk galaxies in early stages of assembly through gas accretion Sects. Low and high-redshift tadpoles show a continuum in their properties Elmegreen and Elmegreen ; Elmegreen et al.
Tadpoles are only a particular sub-set within the BCD family. Therefore, the arguments used for tadpoles supporting gas accretion can be extended to BCDs as well. Star formation in galaxies is self-regulated by the intimate connection between gas mass and SFR, so that the SFR plus outflows driven by feedback tend to balance the cosmic gas accretion rate Sects.
These inflows and outflows controlling galaxy evolution show up as intervening absorption systems along the lines-of-sight to background sources, typically QSOs. All of these features are present in the observed absorption systems. DLAs are predominantly neutral gas reservoirs with metallicities that decrease with increasing redshift. A similarly weak evolution with redshift of the metal content of the IGM had been noted by Pettini et al. Ribaudo et al. This was taken as proof of gas infalling onto the galaxy see the discussion in Sect.
From this and other evidence from the literature, Ribaudo et al. In a subsequent work by this team, also mentioned in Sect. The metal-poor gas has properties consistent with cold accretion streams, while the metal-rich gas likely traces winds, recycled outflows and tidally stripped gas. The behavior of the bimodal metallicity is evidence that inflows and outflows do not mix, and that outflows do not prevent inflows, in agreement with numerical simulations Sect.
Multi-temperature gas in galaxy haloes has been reported too. Crighton et al. The CGM of this galaxy seems to be highly inhomogeneous. The majority of the gas is in a cool, metal-poor and predominantly neutral phase, but the majority of the metals are in a highly ionized phase. They detected high and low ionization species. The strong detected OVI seems to arise in interface material surrounding the photoionized clouds responsible for the low ionization absorption. A cold phase has also been found by Giavalisco et al.
Low temperature is not necessarily equivalent to cold accretion. In this case the metallicity is supposed to be high and so the authors infer that the material is not from the cosmic web but from dwarf satellites or galactic fountains. When several lines-of-sight to QSOs are present near a galaxy, the halo gas can begin to be mapped. MgII absorption lines are detected along each of the four sightlines in both galaxies. A comparison of the linewidths and velocities with that of their galaxy disks rules out a disk origin or a wind outflow. Instead, the MgII line kinematics are consistent either with infalling streams from the cosmic web or from tidally stripped gas.
One of the predictions of cosmic gas accretion refers to very massive galaxies and the quenching of star formation. If a galaxy becomes too massive, the cosmic web gas is shock-heated when entering the halo, and it takes a long time before the gas settles down and can be used to form new stars Sects.
Ionization models suggest a cold gas structure surrounded by a hot cloud. The authors interpret the HI complex as a metal-poor filamentary structure being shock heated as it accretes into the halo of the galaxy. The source of this neutral gas would be the ionized IGM that contains most of the baryons e. Cold-flow streams contain partly ionized gas undergoing continuous recombination and should produce a hydrogen emission line spectrum Sect.
The number of known LABs is still not very large. Their shapes range from circular to filamentary. The LAB sample shows a possible morphology—density relationship: filamentary LABs are in average density environment while circular ones reside in both average and overdense environments. Matsuda et al. The large distance between the two QSOs and the very broad morphology of the nebula argue against the possibility that it originates from an interaction between the two QSO host galaxies.
The work and the figure are from Cantalupo et al. So far we have been citing evidence for gas accretion driving SF. The question arises as to what is the actual contribution of this cosmic web gas to the observed SF; that is, what is the fraction of SFR in gas that has not previously been inside other stars compared to SFR in gas that has been reprocessed in the galaxy? From a theoretical point of view, the fraction of recently accreted gas involved in star formation must be very high. Theory also tells us that the fraction depends very much on the galaxy tendency to lose mass through winds, so that the larger the losses the higher the fraction of cosmological gas needed to maintain the SF.
This section collects a few estimates of this fraction, i. The list is not exhaustive but the number is limited mainly because there are only a few estimates of this parameter in the literature. In general, observations support the theoretical expectations, but the issue is far from settled. It is corrected for Malmquist bias; therefore, they represent the SF in a fixed volume. Taken from Kaviraj They estimate the evolution of stellar mass in these systems from the observed SFRs and the amount of stellar and gas mass added due to observed major and minor mergers.
The measured gas mass is insufficient to maintain the inferred star-formation history, and the needed additional gas mass cannot be accounted for by gas delivered through minor and major mergers or by recycling of stellar ejecta. Numerical simulations predict that the accretion of metal-poor gas from the cosmic web drives the growth of disk galaxies.
They describe galaxies as open systems where gas inflows and outflows determine the gas available to form stars and therefore the global properties of the stellar populations. Observational evidence for gas accretion in galaxies is also numerous although indirect. The gas that falls in or goes out is tenuous, patchy, partly ionized, multi-temperature, and large scale; therefore, it is hard to show in a single observation. A multi-wavelength multi-technique approach is often needed to formulate a compelling case for gas flows associated with SF.
Among the many examples given in this paper, we have selected three cases for the sake of illustration: 1 the short gas-consumption timescale compared to the long SF record in most galaxies Sect. The global picture of the interplay between cosmic gas accretion and star formation seems to be understood at a basic level Sect. In the long term, the gas accretion rate has to balance the SFR since the gas available to form stars is consumed quickly. This balance is self-adjusted by the galaxy modifying the mass of gas available to form stars which, through the KS relation, sets the SFR.
The details of the process remain to be worked out, though. They depend on the coupling between physical processes that involve completely different physical scales: from cosmic web structures to molecular clouds and AGN engines.
Many ingredients needed to complete the observational picture remain to be gathered and understood. For example, the large scale of the gas in the cosmic web should be imaged. Using SFR as a proxy for gas accretion rate, star-forming galaxies can be used to map the gas accretion rate in the universe, near and far. The FMR provides a diagnostic potential that has not been fully exploited yet. Assuming that it is set by cosmic gas infall triggering SF Sect.
This is a venue to be explored very much in the vein of the work by Forbes et al. This poses a problem. Several possibilities have been explored e. Details of the interaction between a cold metal-poor gas stream and a galaxy disk remain to be modeled.
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Predictions on how this interplay excites starbursts and their metallicities are needed to secure our understand of the FMR Sect. Outflows from dwarfs seem to be the main source of metals in the IGM. However, the mixing processes of these outflows with the IGM are poorly captured by numerical simulations.
The cooling of halo gas onto supernova debris in galactic fountains has also been suggested as a source of the metal-poor gas driving SF in the MW and other galaxies Sect. Can this possibility be proved or disproved? Cosmic web gas might also be detected through the Sunyaev—Zeldovich effect imprinted on the Cosmic Microwave Background; this is a promising technique under development now e. These and other future research programs will contribute to our understanding of cosmic accretion. They will eventually complete the picture that has been emerging over the last few years, in which cosmic gas accretion plays a fundamental role in sustaining star formation in disk galaxies.
It turns out to be roughly comparable in luminosity to the MW. Daniel Ceverino helped us in dissecting the predictions of the cosmological numerical simulations, and Sects. Discussions with Claudio Dalla Vecchia were also fundamental for the development of many aspects of the work. Fernando Atrio pointed out the work to image the cosmic web through the Sunyaev—Zeldovich effect.
Star formation sustained by gas accretion
Figures 6 and 7 have been reproduced with permission of the AAS, and Figs. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author s and the source are credited. Skip to main content Skip to sections. Advertisement Hide. Download PDF. Star formation sustained by gas accretion. Open Access.
First Online: 16 July The gas originally resides outside the virial radius of the dark-matter DM halo that hosts the galaxy, and by accretion over cosmic time it becomes part of the pool of baryons that forms new stars. This cosmological gas supply has a strong dependence on redshift and halo mass. For less massive haloes, cool gas streams can reach the inner halo or disk directly.
This so-called cold-flow accretion may have an extreme impact on the disk, or it may provide a gentle gas supply that is transported radially in the disk. Since high-redshift haloes tend to be low in mass, cold-flow accretion is predicted to be the main mode of galaxy growth in early times. Ultimately, the galaxies evolve into a quasi-stationary state Sect. Open image in new window. This paper is organized as follows: Sect. Evidence from stellar properties is included in Sect. A number of observational properties characterizing large samples of star-forming galaxies can be explained if the SF is driven by metal-poor gas accretion.
These properties are put forward and discussed in Sect. Theory predicts gas accretion to be particularly important at high redshift. In addition, it treats the secondary role of mergers Sect. Theory predicts that most of the gas going into stars in a typical SF episode is not recycled gas from previous star formation, but it comes from accretion.
The review concludes by summarizing the role of gas accretion in star formation, and indicating several open issues to be explored in the future Sect. Table 1 List of main acronyms and symbols defined and used along the text. At early times, when galaxies were generally of low mass, and at recent times in the case of low-mass galaxies, a significant fraction of the gas from cosmological accretion remains cold and falls directly to the center Birnboim and Dekel ; Semelin and Combes ; Stewart et al.
Using equations from chemical evolution models e. The mass in metals can be expressed using a differential equation similar to Eq. Average mass infall rate. Cosmological accretion drives star formation. It is primarily dark-matter accretion carrying along baryons—intermittent but with a marked global trend. A summary of numerical model predictions is shown in Fig.
This cold component comes directly from the intergalactic medium IGM and is more metal poor than the hot component Fig. The gas falls in pulled by gravity so that the characteristic velocities are similar to the Keplerian velocities in the outer parts of disks.